Re: Safe enough - IF no third party code
If an engineer at the company I work for were to create such a backdoor in software that shipped to a customer they wouldn't just be looking a dismissal, they'd probably be looking for a lawyer to keep them out of jail.
Agreed. The days of hardcoding a special field service password or Nintendo-style backdoor into enterprise hardware is over, the publicity if it is found out would be a killer - consider that for it to be useful your employees (some of whom will eventually become ex-employees) have to know about it!
Besides even the built-in backdoors that used to be common were just a way to get in as 'admin' (or sometimes "admin plus") if your customer forgot the password. Even if they dropped you down to some sort of shell, there's a long way from that to having the proper build environment to compile something that will run on it.